I've always wondered when and why would you use pipe and when and why would you use tubing?

Pipe is a vessel for carrying fluids. It is made to an INSIDE measurement and this measurement should remain consistent regardless of the outside measurement.

Tube is a structural member made to carry loads and is manufactured to the OUTSIDE measurement and a load rating which will remain consistent.

Technically speaking you should be using tube to build stuff with and pipe to carry the water/brake fluid etc. but that doesn't stop people interchanging uses without a problem if you know the limitations.